Enhancement mode (E-mode) devices based on GaN technology are interesting for power system applications. Utilization of a p-type GaN or a p-type AlGaN layer under the gate metal depletes the channel so as to achieve normally-off operation.
In a typical HEMT device, a blanket Mg-doped p-type GaN or a blanket Mg-doped p-type AlGaN layer is grown on top of an AlGaN barrier. A selective p-GaN to AlGaN etch is used to fabricate the p-type GaN gate and to remove the p-GaN layer in the access region positioned between a gate and a source/drain. Etching of p-GaN selective to AlGaN is difficult and furthermore leaves the surface between the gate and the source/drain damaged. Moreover, during growth of the p-GaN layer, Mg diffuses into the AlGaN/GaN stack. As a result, the device architecture is subject to a high resistive 2DEG access region and dispersion, which leads to current collapse. The blanket p-GaN growth also limits the threshold voltage (VTH) of the device.
An alternative approach is presented a European patent application, EP2 602 827 A2. In this patent application, a selective regrowth of the p-GaN layer in a recessed gate region is described, where an in-situ grown SiN layer is present in the access region. Although this approach may solve potential dispersion issues and avoid Mg diffusing into the access region thanks to the presence of the SiN in the access region, this method is subject to loading effects during regrowth of the p-type GaN layer, which make the growth rate, the thickness and the doping concentration of the regrown p-GaN layer dimension dependent. A high growth rate of the p-GaN layer in the gate area results in reduced incorporation of Mg into the p-GaN layer.
A non-selective regrowth of p-GaN layer in the recessed gate region and in the access region is also disclosed in the same European patent application, EP2 602 827 A2. However, the p-GaN layer regrown by non-selective deposition is likely to suffer from low threshold voltage compared to that obtained by p-GaN regrown by selective growth since temperature of non-selective growth is reported to be lower that the temperature of selective growth.
An enhancement mode Group III/nitride HEMT provided with more flexibility in design to obtain high VTH, high drain current (IDS) and with reduced loading effects may be useful.